Crystalline plastics such as POM (Polyoxymethylene) have a shrinkage rate of 1.5%-3% due to ordered molecular arrangement; amorphous plastics like PC (Polycarbonate) have a shrinkage rate of 0.5%-1.5%. Hygroscopic materials such as nylon will expand by 0.2%-0.5% in environments with humidity exceeding 60%, requiring attention to environmental humidity control.

The machining tolerance of cavities and cores of high-precision molds should be controlled within ±0.01mm, ensured by processes such as wire electrical discharge machining. After long-term use, if the parting surface wears by more than 0.03mm, flash will occur; unreasonable design of cooling water channels will cause uneven cooling and local dimensional deviations of plastic parts.

II. Testing Preparation Requirements

The testing environment should comply with ISO 291 standards, with temperature at 23°C±2°C and relative humidity at 50%±5%. For PC materials, a temperature change of 5°C causes a deviation of 0.05%-0.1%; for nylon, a humidity change of 10% leads to a fluctuation of 0.1%-0.3%. A constant temperature and humidity chamber should be equipped and calibrated monthly to ensure temperature uniformity ≤±0.5°C and humidity uniformity ≤±3%.

After demolding, samples should be placed in the testing environment for 2 hours. Key dimensions should be measured with coordinate measuring machines, with each dimension measured 3 times to take the average. Data should be retained to four decimal places, and environmental parameters recorded to establish a database.